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I'm sorry I can't give you the fix but I think it points to Veritas side more than the Oracle side (only my opinion)

Couple of thoughts:

Have you taken backups with Veritas at this time of day before? - Within Veritas you can configure a backup window and any backup started outside this window will fail. This could result in your error as Oracle would not be able to write the require info. Do you have a backup window?

If I remember correctly there is also a setting that will prevent a backup running after x minutes of the last backup. So if this is set to 600 then once you've taken a backup you can't take another one through Veritas for 10Hrs (600 Mins).

Can you use Veritas to write to the tapes without using RMAN?
If you can't then it's probably a problem with Veritas and not Oracle

If you can then has anything changed in the Veritas/Oracle configuration recently?